The invention relates to an optically variable surface pattern of the kind set forth in the classifying portion of claim 1.
Such optically variable surface patterns with a microscopically fine relief structure are suitable for example for increasing the level of security against forgery and for conspicuously identifying articles of all kinds and can be used in particular in relation to value-bearing papers or bonds, identity cards, payment means and similar articles to be safeguarded.
A surface pattern of the kind set forth in the classifying portion of claim 1 is known from EP 375 833. The surface pattern which is embossed in the form of a light-modifying relief structure into a carrier is subdivided into grid areas. Each grid area is divided into a number n of surface portions, wherein each surface portion is associated with a pixel of one of n representations and wherein each has a respective diffraction element which contains items of information about a chromaticity, a brightness value and a viewing direction. The n representations are composed of beams of diffracted light which become visible at n different viewing directions. In order that a representation becomes visible only at a single viewing direction the corresponding relief structures are of an asymmetrical profile shape.
EP 360 969 discloses a diffraction element which has surface portions with colours of high luminosity. The surface portions contain relief structures which are in the form of diffraction gratings with an asymmetrical profile shape, for example with a sawtooth-shaped profile configuration. The diffraction gratings reflect incident light predominantly in the first diffraction order. For that reason the diffraction gratings change their colour with a varying direction of incidence of the light and a varying direction of view on the part of an observer. The achievable degree of asymmetry, that is to say the ratio of the level of intensity of the light diffracted into the plus first diffraction order to the intensity of the light diffracted into the minus first diffraction order is typically 3:1 and at most 30:1.
DE 25 55 214 discloses optical markings which modify incident light essentially not by diffraction but by reflection or optical refraction on the basis of the laws of geometrical optics. With line spacings of 10 to 100 microns however those configurations already give profile heights of several or several tens of micrometres, at moderate reflection angles.
It is known from the technical literature, for example from the book "Diffraction Gratings", M. C. Hutley, Chapter 2, pages 13-56, ISBN 0-12-362980-2 that light of a wavelength .lambda. which is incident on a grating structure from a direction of incidence is diffracted in accordance with the following relationship: EQU sin(.theta..sub.m)=sin(.theta..sub.i)+m*.lambda./d (1)
wherein d denotes the grating period, .theta..sub.m and .theta..sub.i denote the intermediate angles between the line normal to the surface with the grating structure and the diffracted beam m and the incident beam i respectively and the integral index m denotes the diffraction order. There are only a finite number of diffraction orders. Accordingly polychromatic light is resolved by the grating structure into its spectral colours, that is to say light of different wavelengths is diffracted into different directions. Now various methods are known for diffracting the light of different wavelengths into the same direction in order within certain limits to avoid spectral colour resolution which is perceptible by the eye and thereby to achieve an achromatic impression. They are based on using grating structures with different grating periods. For example it is possible for grating structures with grating periods d.sub.1, d.sub.2 and d.sub.3 to be arranged in mutually juxtaposed relationship in grid areas. The size of the grid areas is so selected that the grid areas are not separately perceptible by the human eye from a normal viewing distance of 30 cm. The periods d.sub.1, d.sub.2 and d.sub.3 of the gratings are so selected that the spectra thereof are in superposed relationship in a predetermined viewing direction, more specifically in such a way that the diffraction directions of the red spectral component of the grating structure 1, the green spectral component of the grating structure 2 and the blue spectral component of the grating structure 3 are the same for a diffraction direction. The individual grating structures do not have to be arranged in mutually juxtaposed relationship but they can also be in mutually superposed relationship as for example in the case of holograms. Juxtaposition can also be replaced by a local, repetitive variation of the grating constant: the surface which is to appear achromatic is subdivided into individual surface portions whose dimensions are below the resolution limit of the human eye. Within a surface portion the local grating period (line spacing) varies in accordance with a predefined, for example linear function, over a given period range. It is further known in regard to an achromatic hologram for the grating period to be locally stochastically altered, see for example the book "Optical Holography", edited by P. Harriharan, Cambridge Studies in Modern Optics, pages 144 ff, ISBN 0 521 31162 2.
All those methods suffer from the common disadvantage that, although an achromatic impression can admittedly be produced for a predetermined viewing angle, pronounced colour fringes appear in the adjoining viewing angles. If moreover the viewing range over which a representation is to appear achromatic is increased by a large period extent, the brightness which can be perceived by an observer decreases noticeably as the incident light is distributed over a correspondingly larger angular range.